Measurement of Gross Photosynthesis, Respiration in the Light, and Mesophyll Conductance Using H₂¹⁸O Labeling

Using a reaction‐diffusion model to estimate day respiration and reassimilation of (photo)respired CO2 in leaves

“…(), who showed that R dk was higher at 21% than at 2% O 2 in developing leaves of Vicia faba . Respiration is a process where O 2 is the substrate (Tcherkez et al ., ) and respiratory rates measured in terms of O 2 and CO 2 exchange may not be equal (Gauthier et al ., ). However, the amount of respiratory CO 2 release at the low O 2 concentration, as applied for measurements under nonphotorespiratory conditions, will likely decrease relative to that under ambient O 2 conditions.…”

“…Techniques have been developed to measure directly (Loreto et al ., , ; Pärnik & Keerberg, ; Gong et al ., ; Tcherkez et al ., ; Gauthier et al ., ), but these techniques usually require access to sophisticated and expensive isotope discrimination measuring devices and are often unavailable. Methods exist to indirectly estimate in C 3 leaves from conventional gas‐exchange measurements (Kok, ; Laisk, ; Brooks & Farquhar, ), sometimes combined with chlorophyll fluorescence measurements (Yin et al ., ).…”

Photosynthetic Oxygen Production: New Method Brings to Light Forgotten Flux

“…In this issue of Plant Physiology, Gauthier et al (2018) remind us why it is so important to return our attention to O 2 , providing us with a new, elegant open-path system to measure O 2 fluxes. Their method is a "reverse" isotopic approach, involving 18 O-labeling of leaf water rather than the air so that the isotopic composition of O 2 that is evolved during water splitting has a signature very different to that of ambient O 2 (Fig.…”

“…This may lead to an overestimate of electron transport rate when computed from fluorescence compared Figure 1. Simple representation of the reactions that can be involved in gross O 2 production and uptake of a photosynthesizing cell, showing how labeled 18 O water results in the production of 18 O 2 in the approach developed by Gauthier et al (2018). In the case of reactions within the peroxisome and mitochondria, this only represents net O 2 consumption, i.e.…”

“…with measurements of GOP. Furthermore, accurate determination of J A is particularly relevant for the estimation of mesophyll conductance, which was one application highlighted by Gauthier et al (2018). The Mehler-peroxidase reactions, which have been shown to range from 0% to 30% (Driever and Baker, 2011), would lead to an overestimate of electron fluxes associated with the photosynthetic carbon reduction/ oxygenation cycles in both methods.…”